1. Field of the Invention
The present invention relates to a thin-conductor highly-foamed-polyolefin-insulated wire for high-speed data transmission, a multi-core cable comprising the wires thereof, and a manufacturing method of the wire and cable.
2. Description of the Background Art
To achieve a higher foaming degree in the insulation of a foamed-polyolefin-insulated wire for high-speed data transmission, published Japanese patent Tokukosho 61-11412 disclosed a method of using a plastic material having a swelling ratio of 55% or more to fulfill a foaming degree as high as 60% or more. Published Japanese patent Tokukosho 63-56652 disclosed a method of foaming a blend of an ethylene-propylene elastic copolymer, high-density polyethylene, and an ethylene-propylene block copolymer to attain the same result.
The drawback of these conventional methods, which apply the use of a material having a specific swelling ratio or the use of a blend of an ethylene-propylene elastic copolymer and others, is that a weak bonding develops between the insulation layer and the conductor when applied to the production of a thin-conductor foamed-polyolefin-insulated wire, causing the stripping length of the insulation to fluctuate undesirably at the time of termination work. A thin-conductor insulated wire innately has a weaker bonding between the insulation and the conductor due to its smaller contact area between them. A higher degree of foaming in the insulation further weakens the bonding, giving the above-mentioned undesirable result.
In order to strengthen the bonding between the insulation and the conductor, published Japanese patent Tokukosho 48-42314 disclosed a method where a thin unfoamed (solid) insulation layer is applied directly over the conductor before a thin foamed-polyethylene insulation layer is applied. This method, however, has difficulty in producing a highly foamed insulation layer for a thin-conductor wire with a good appearance. In order to keep the same total insulation thickness, an unfoamed insulation layer on the conductor requires a reduction of the foamed insulation thickness. To obtain the same equivalent dielectric constant, the foamed insulation needs to have a higher foaming degree. An extrusion of an insulation layer with a thinner thickness and a higher degree of foaming makes it difficult to produce a thin-conductor highly-foamed insulated wire.
In order to strengthen the bonding between the insulation and the conductor, published Japanese patent Tokukohei 6-16371 disclosed a method where a conductor is cooled just before entering an extruder for producing a foamed-polyolefin-insulated wire. This method too cannot produce a required high degree of foaming for a thin-conductor foamed-polyolefin-insulated wire. In this method, the insulation around the conductor is cooled rapidly to suppress the foaming so that the bonding with the conductor will be enhanced. As with the method of applying a solid insulation layer on the conductor described above, the foamed insulation layer needs to have a higher foaming degree to compensate the higher dielectric constant of the solid insulation layer, making it difficult to produce a thin-conductor highly-foamed thin-insulation wire.
In actual application, increasingly thinner cables are being used for data transmission as in computers to save wiring space, for example. Because the data transmission speed is inversely proportional to the square root of the dielectric constant of an insulation layer, a foamed insulation is used to reduce the dielectric constant. The higher the foaming degree, the higher the transmission speed. For a thick-conductor less-foamed insulated wire, the large contact area between the foamed insulation layer and the conductor facilitates a strong bonding between them.
On the other hand, for a thin-conductor highly-foamed insulated wire, the smaller contact area between the foamed insulation layer and the conductor makes it extremely difficult to achieve a strong bonding between them. If the bonding is weak, when a wire is cut, the conductor will protrude from the cut end, and when the insulation is stripped, the stripped length will fluctuate undesirably. These phenomena are particularly notable in a foamed insulation wire less than 0.4 mm in conductor diameter and 50% or more in foaming degree.
In addition, when the diameter of a foamed insulation layer decreases, it becomes a considerable challenge to obtain a good appearance of the extruded layer. In many cases the foamed insulation layers have a coarse appearance when the insulation thickness is less than 0.8 mm.
As a result of intensive studies on the above-mentioned problems, the present inventors have found the following facts and completed the present invention.
A highly-foamed-polyolefin-insulated wire which has a conductor of 0.4 mm or less in diameter and an insulation of 0.8mm or less in thickness and which satisfies the following three conditions is suitable for high-speed data transmission:
(a) an equivalent dielectric constant of less than 1.6;
(b) the fluctuation limits of the equivalent dielectric constant of xc2x10.1; and
(c) a conductor stripping force of 100 g/50 mm or more.
The abovementioned insulated wire can be obtained by using polyolefin including 10 weight % or more of a partial metal salt of a copolymer having a comonomer containing carboxylic acid or a carboxylic anhydride group. It is desirable that in the above-mentioned insulated wire, polyolefin having a swelling ratio of 55% or more be extruded over the conductor to form a foamed insulation layer with a foaming degree of 50% or more using a chemical foaming agent and/or an inert gas.